The present invention relates to chromatic dispersion in optical networks, and more particularly to compensating for chromatic dispersion in optical networks.
Fiber optic networks are becoming increasingly popular for data transmission because of their high speed and high data capacity capabilities. Wavelength division multiplexing is used in such fiber optic communication systems to transfer a relatively large amount of data at a high speed. In wavelength division multiplexing, multiple information-carrying signals, each signal comprising light of a specific restricted wavelength range, may be transmitted along the same optical fiber.
In this document, these individual information-carrying lights are referred to as either xe2x80x9csignalsxe2x80x9d or xe2x80x9cchannels.xe2x80x9d The totality of multiple combined signals in a wavelength-division multiplexed optical fiber, optical line or optical system, wherein each signal is of a different wavelength range, is herein referred to as a xe2x80x9ccomposite optical signal.xe2x80x9d
One common and well-known problem in the transmission of optical signals is chromatic dispersion of the optical signal. Chromatic dispersion refers to the effect wherein the lights of different wavelengths comprising an optical channel travel through an optic fiber at different speeds. For optical fiber, chromatic dispersion is defined by the quantity D (ps-kmxe2x88x921-nmxe2x88x921) through the relationship of Eqn. 1                     D        =                                            ⅆ                              ⅆ                λ                                      ⁢                          (                              1                                  v                  g                                            )                                =                                    1              L                        ⁢                          (                                                ⅆ                                      τ                    g                                                                    ⅆ                  λ                                            )                                                          Eqn        .                  xe2x80x83                ⁢                  (          1          )                    
In the above eqn. 1, the quantity xcex is the physical wavelength of signal light (nm), xcexdg is the group velocity (km/ps) of the signal light at the wavelength xcex, L is the fiber length (km) and xcfx84g is the delay time (ps) required for light of wavelength xcex to propagate the distance L. If xcexdg decreases with increasing wavelength (i.e., longer or xe2x80x9credxe2x80x9d wavelengths travel slower than relatively shorter or xe2x80x9cbluexe2x80x9d wavelengths) then D is positive, otherwise D is negative. The quantity D is an intrinsic property of each fiber type and may vary with wavelength. The related quantities Df and Dc are herein defined by Eqn. (2)                               D          f                =                  LD          =                                    (                                                ⅆ                                      τ                    g                                                                    ⅆ                  λ                                            )                        =                          -                              D                c                                                                        Eqn        .                  xe2x80x83                ⁢                  (          2          )                    
wherein Df is the time delay, per unit change in wavelength, produced by a length L of fiber and Dc is the opposite time delay, per unit change in wavelength, which must be produced by a dispersion compensator so as to exactly compensate for the fiber""s chromatic dispersion.
The problem of chromatic dispersion becomes more acute for data transmission speeds higher than 2.5 gigabytes per second. The resulting pulses of the signal will be stretched, will possibly overlap, and will cause increased difficulty for optical receivers to distinguish where one pulse begins and another ends. This effect seriously compromises the integrity of the signal. Therefore, for a fiber optic communication system to provide a high transmission capacity, the system must compensate for chromatic dispersion. The exact value of the chromatic dispersion produced in a channel of a wavelength-division multiplexed fiber optic communications system depends upon several factors, including the type of fiber and the wavelength of the channel.
Accordingly, there exists a need for a tunable chromatic dispersion compensator. The present invention addresses such a need.
The present invention provides an improved tunable chromatic dispersion compensator. The compensator includes: a virtually imaged phased array (VIPA); at least one reflector optically coupled to the VIPA; a mirror optically coupled to the at least one reflector; and a movable reflector holder coupled to the at least one reflector, where the movable reflector holder moves the at least one reflector such that a length of a beam path between the VIPA and the at least one reflector and the mirror is variable. The present invention uses the VIPA to produce a controlled variable degree of chromatic dispersion within a plurality of optical channels so as to compensate for unwanted chromatic dispersion in an optical communications system. Positional adjustment of the movable reflector holder permits variable control of the beam path length between the VIPA and the focusing lens. This variable change in beam path length permits variable control of the magnitude and sign of chromatic dispersion provide by the compensator.